1. Field of the Invention
The present invention relates to a magnetic recording drive, a magnetic recording medium and a method for manufacturing the same and, more particularly, to a magnetic recording drive for use in an external memory device of an information processing apparatus, etc., a magnetic recording medium used therein and a method for manufacturing the same.
2. Description of the Prior Art
In the magnetic recording drive, improvement of the recording density has been demanded more and more with an increase of an amount of information in proportion. When recording density of the conventional magnetic recording medium is increased, a S/N ratio is degraded to cause reduction of a reproducing output and increase of noise. Therefore, the magnetic recording medium enabling a large reproducing output and low noise has been demanded.
In particular, the problem is to achieve noise reduction in the magnetic recording medium since reading sensitivity has been extremely improved by practical use of the magnetoresistance head.
As a main factor of generating the noise in the magnetic recording medium, there is unclear boundary of the magnetization transition regions due to variation in magnetization in magnetization transition regions. The variation in magnetization is caused by magnetic interaction between crystal grains of the ferromagnetic film constituting the ferromagnetic layer.
In order to reduce noise in the magnetic recording medium, it is required to weaken the magnetic interaction between crystal grains of the ferromagnetic film.
In general, as the recording layer of the conventional magnetic recording medium, a thin film which is formed of cobalt (Co)-based ternary or quaternary alloy by sputtering may be used. By adjusting composition of the thin film and manufacturing conditions, segregation in the ferromagnetic portion and the nonmagnetic portion can be facilitated to reduce the noise.
In the conventional magnetic recording medium, as shown in FIG. 1, for example, a chromium layer 2, a magnetic recording layer 3 consisting of CoCr12Ta2, and a protection layer 4 consisting of a carbon film are formed in that sequence on a nonmagnetic substrate 1 which is formed of an Al substrate covered with a NiP film.
However, since a cobalt system alloy constituting the recording layer 3 is inherently a solid solution, it is difficult to isolate crystal grain of the ferromagnetic film perfectly even if segregation is accelerated by adjusting composition and manufacturing conditions.
As the way of isolating crystal grains of the magnetic substance. Patent Application Publication (KOKAI) JP59-42642 and Patent Application Publication (KOKAI) P59-220907 have set forth manufacturing methods such that a binary or ternary alloy layer comprising the nonmagnetic substance such as silver and copper and the ferromagnetic film which is insoluble in this nonmagnetic substance is once formed by sputtering, and then the alloy layer is heated.
In the manufacturing methods set forth in these publications (KOKAIs), the ferromagnetic layer is heated at a temperature of less than 400xc2x0 C. to accomplish high coercive force. Since a glass or polymer film is utilized as a substrate for supporting the magnetic recording film, the heating temperature of less than 400xc2x0 C. is preferable.
In both Publications, the manufacturing methods for forming the magnetic recording film which has film thickness of 130 to 150 nm and txe2x80xa2Br value of 2000 Gaussxe2x80xa2xcexcm have been set forth. The txe2x80xa2Br value is denoted as a product of residual magnetization Br and a film thickness t of the magnetic recording medium (magnetic recording film).
However, in the magnetic recording medium for use in the magnetoresistance head, it is requested that the thickness of the magnetic recording layer would be set to be lower than or equal to 30 nm and also the txe2x80xa2Br value would be set to be lower than or equal to 150 Gaussxe2x80xa2xcexcm. In the case of manufacturing of the magnetic recording layer, the techniques set forth in these publication cannot be applied as they are. This is because of the following reasons.
First, the relation between recording density and effective output voltage in the magnetic recording medium for use in the magnetoresistance head has been well known as described in FIG. 2.
In FIG. 2, in case the recording density is small like about 10, 20 kFRPI, the effective output is increased if the txe2x80xa2Br value is increased. But, in case the recording density is large like about 50, 100 kFRPI, the effective output voltage is decreased when the txe2x80xa2Br value is increased.
For this reason, if the recording density is increased up to about 50, 100 kFRPI, the txe2x80xa2Br value of the magnetic recording layer must be set to be lower than 150 Gaussxe2x80xa2xcexcm.
However, even if, under the conditions set forth in the above publications, it has been tried to accomplish the txe2x80xa2Br value of less than 150 Gaussxe2x80xa2xcexcm by forming the magnetic recording layer of less than 30 nm in thickness. However, even under these conditions, noise reduction and large coercive force have not been achieved since crystal grains in the magnetic recording layer are small and further partially continuous with each other in such circumstances.
Next, it can be considered that the magnetic recording layer 3 formed of CoCr12Ta2 is formed thinner. For example, as shown in FIG. 1, the chromium layer 2 of 100 nm in thickness and the magnetic recording layer 3 of 20 nm in thickness are formed in that order on the nonmagnetic substrate 1 formed of a two-layered structure consisting of Al and NiP, and then the protection layer 4 formed of carbon is formed thereon to have a thickness of 20 nm. At this time, the txe2x80xa2Br value is about 100 Gaussxe2x80xa2xcexcm.
While the relation between the recording signal frequency and noise power in the magnetic recording layer has been investigated using the magnetoresistance reproducing head, the result has been derived as shown by the broken line in FIG. 9. It has been appreciated that noise power is increased linearly in proportion as the recording signal frequency is increased. As a result, it has been found that the thinned CoCr12Ta2 is not fit for the magnetic recording layer used for high recording signal frequency.
In the examination in FIG. 2, a relative velocity between the magnetic head and the magnetic recording medium is elected as 10 m/s, the recording signal frequency is set to 20 MHz, and the recording density is selected as about 100 kFRPI.
As has been stated above, regarding a granular magnetic film (Fe-SiO2) in which magnetic fine grains are dispersed into the SiO2 film, the following problem is caused in addition to the problem of the magnetic characteristic due to crystal property of the magnetic recording film.
It has been recited in Applied Physics Letter, 52 (6), 512 (1988) and U.S. Pat. No. 4,973,525 that, in the above granular magnetic film, crystal property of the magnetic substance fine grains has been improved and also more preferable magnetic characteristics and recording/reproducing characteristics could be achieved by controlling the substrate temperature appropriately at the time of film-formation.
Thereby, there is a tendency that, in the granular magnetic film, segregation of the magnetic substance has too small size in the state of as-grown to show enough coercive force. It has been seen that, in order to increase coercive force, the annealing must be effected after growing the film to increase the volume of each segregation.
On the contrary, conventionally a NiP plated substrate has been used mainly as the substrate for the magnetic recording medium. But, the NiP layer formed on a surface of the substrate has been crystallized by heating at a temperature in excess of 300xc2x0 C. Thus, there are caused some problems that flatness of the surface of the layer is damaged, the layer is magnetized, or the like. It is evident that such substrate is not adequate for heating process at a high temperature.
In recent years, it has been considered that, on the trend of downsizing, the magnetic disk is reduced in size to have the same size as the IC card. In this case, a thickness of the magnetic disk must be formed less than 3 mm. In this case, a thickness of the substrate must also be formed less than 0.3 mm, but it is the problem to use the NiP plated substrate in the respect of mechanical strength.
Like this, a glass substrate or a single crystal substrate in place of the Ni-P plated substrate is examined to proceed thinner-layered structure and planarization of the magnetic recording medium. For example, it has been proposed in Patent Application Publication (KOKAI) 59-96538 that the structure in which a chromium (Cr) layer, a magnetic film, and a protection film are grown in sequence on the single crystal substrate may be used as the magnetic recording medium.
From the previous discussion, it is of course necessary to consider that, after the Fe-SiO2 granular magnetic film is grown in the single crystal silicon substrate, the granular magnetic film must be treated by heating process. In that case, it will be supposed that the SiO2 film covering the granule of Fe serves to prevent the reaction between the silicon substrate and the granule.
However, based on the experiments effected by the inventors of the present invention, it has been found that, when the granular magnetic film formed on the silicon substrate is heated at a high temperature, atoms of the magnetic substance and silicon atoms are mutually diffused passing through the grain boundaries in the granular magnetic film, so that paramagnetic silicon compounds are formed to thus reduce the value of saturation magnetization (Ms) of the magnetic recording medium.
Especially, in the magnetic recording medium in which a low txe2x80xa2Br value (where t is a thickness of the magnetic layer, and Br is a magnitude of residual magnetization) is required to be used together with the MR head, it causes a serious problem that the magnetic grain is wasted to form silicon compounds since an amount of the magnetic grain in the magnetic film is absolutely small.
On the other hand, when the magnetic recording medium is formed of a plurality of different material layers and thereafter it is heated, there may be a risk of causing a separation of the layers due to difference between coefficients of thermal expansion of the layers.
A first object of the present invention is to provide a method for manufacturing a magnetic recording medium, which is capable of reducing noise and achieving high coercive force and is fit for a magnetoresistance head.
In the present invention, a nonmagnetic film, a ferromagnetic film and the nonmagnetic film are formed separately on a substrate, then a resultant structure is annealed to distribute crystal grains of the ferromagnetic film into the nonmagnetic film, whereby a recording layer is formed.
Thus, the crystal grains of the ferromagnetic film may be spaced and isolated from each other such that all adjacent crystal grains of the ferromagnetic film do not magnetically interact with each other in the recording layer. In this case, if the nonmagnetic substance in which the ferromagnetic film is scarcely soluble is utilized, the above tendency becomes particularly conspicuous.
Therefore, by making distribution of magnetization uniform in the magnetic recording medium, a noise characteristic can be improved which is caused by uneven distribution of magnetization in magnetization transition regions and their peripheral regions of the magnetic recording medium.
Furthermore, according to the above manufacturing method, such a recording layer can be formed that has a thin film thickness, realizes satisfactory coercive force, and attains 150 Gaussxe2x80xa2xcexcm or less, preferably 100 Gaussxe2x80xa2xcexcm or less in a product of residual magnetic flux density and a film thickness. As a result, large reproducing output which is fit for high sensitivity performance of the MR head can be obtained.
In addition, if the magnetic recording layer is annealed at a high temperature, for example, 400xc2x0 C. or more so as to facilitate mutual diffusion and also to produce crystal structures for generating sufficient magnetization as distributed crystal grains of the ferromagnetic film, further large coercive force can be obtained.
In the above description, as a substance of the ferromagnetic film, cobalt or an alloy including the cobalt as a major constituent, for instance. CoACr100-A (A is 90 or more), CoAPt100-A (A is 70 or more, or 40 to 50) or CoASm100-A (A is 83.3 or 89.5) may be used. As a substance of the nonmagnetic film, metal, oxide, nitride, carbon or carbide may be used.
Moreover, as a substance of the nonmagnetic film, it is preferable to use a substance which has a solid solubility of cobalt of 5% or less, for instance, metal such as silver or copper, silicon oxide or zirconium oxide, titanium nitride or silicon nitride, carbon or carbide, or the like.
Also, a high heat-resistant material, for instance, silicon or carbon, is suitable for a substrate of the nonmagnetic substrate.
In addition, in the present invention, the magnetic recording layer of an alloy consisting of the ferromagnetic film and the nonmagnetic substance is formed on the heat-resistant nonmagnetic substrate to have a thickness of 30 nm or less, and then the magnetic recording layer is annealed during it is being formed or after it is formed, whereby plural isolated grains formed of the ferromagnetic film and having an average grain diameter of 50 nm or less are obtained.
In the magnetic recording layer thus formed, since the grains of the ferromagnetic film can be formed to have large size and isolated from each other, variations in magnetization in magnetization transition portions can be suppressed. Therefore, in contrast to the conventional device, noise can be reduced upon recording or reproducing signals, and dependency of noise power on recording signal frequency can also be eliminated.
Also, in the case in which the magnetic recording layer having a film thickness of less than 30 nm is formed, it has been experimentally confirmed that coercive force of the magnetic recording layer may be increased if an annealing temperature is set at 400xc2x0 C. or more, preferably in a range of 400xc2x0 to 550xc2x0 C. while coercive force may be increased if the annealing temperature is set at 400xc2x0 C. or less.
Accordingly, the magnetic recording layer having high signal quality (high S/N and high recording density may be obtained.
Besides, if a txe2x80xa2Br value of the magnetic recording layer is set lower than or equal to 150 Gaussxe2x80xa2xcexcm, the magnetic recording layer is most suitable for the magneto-resistance head.
A second object of the present invention is to provide a magnetic recording medium capable of preventing a separation of layers constituting it and suppressing degradation of coercive force by preventing a reaction between silicon in the silicon substrate and magnetic substance in a granular magnetic film, and a method for manufacturing the same, and a magnetic recording device utilizing such magnetic recording medium.
According to the present invention, the nonmagnetic layer including no magnetic grain therein is formed between the nonmagnetic layer including magnetic grains therein and the silicon substrate. The nonmagnetic layer including the magnetic grains therein serves as the granular magnetic layer, and the nonmagnetic layer including no magnetic grain therein serves as a diffusion preventing layer. Thus, mutual diffusion of the silicon in the silicon substrate and the magnetic grain due to annealing may be prevented by the diffusion preventing layer.
Therefore, since the silicon containing the magnetic material is not formed, reduction of the magnetic material may be prevented so that the magnetic recording medium having large coercive force and high recording density can be achieved. In addition, since the granular magnetic layer and the diffusion preventing layer excepting the magnetic fine grains are formed by the same substance, difference in thermal stress does not occur in the granular magnetic layer and the diffusion preventing layer. Thus, a stable layer structure can be obtained without the possibility of film exfoliation. A silicon oxide film, for example, can be listed as the nonmagnetic layer. There are iron, cobalt and nickel, for example, as the magnetic grain. Further, an adhesion between a silicon dioxide layer and a silicon substrate is extremely superior and there is no risk of a separation therebetween.
Besides, it may be considered to form the diffusion preventing layer with nonmagnetic material which has the same coefficient of thermal expansion as the nonmagnetic material constituting a granular magnetic layer and the different composition from it. However, it requires much labor that two kinds of nonmagnetic material need to be separately alloyed. In order to avoid this, it is preferable that their kinds are same.
If a product (txe2x80xa2Br) of a thickness (t) and residual magnetization (Br) of the granular magnetic layer is set at 100 Gaussxe2x80xa2xcexcm or less in such structure, the magnetic recording medium which is most suitable for signal detection by the magnetoresistance head can be derived.
Incidentally, in the case the silicon substrate is heated, it is neither magnetized nor deformed even if it is heated at a temperature in excess of 300xc2x0 C., for example, 1000xc2x0 C.